It was recognized by early workers that coal can be liquified by controlled heating in the substantial absence of oxygen. The conversion products are a liquid and a char. Because of the new compelling economic factors, the technology of coal liquefacation and gasification has been expanding at an accelerated pace. Pioneer developments in the field are represented by Lurgi and Fischer-Tropsch technology. More recent advances in coal liquefaction are described in United States Patents Nos. 1,904,586; 1,955,041; 1,996,009; 2,091,354; 2,174,184; 2,714,086; 3,375,188; 3,379,638; 3,607,718; 3,640,816; 3,642,608; 3,705,092; 3,849,287; 3,870,621; inter alia.
Hence, by recently developed methods, coal can be converted into hydrocarbon liquids by subjecting coal to reaction with solvents, with or without hydrogen, to yield a slurry mixture of dissolved coal and a residuum of unreacted coal and mineral ash. The solids are present in the slurry as a finely divided suspension of insoluble matter.
The suspended solids have a high content of ash which reduces the value of the slurry as a fuel and limits its use to special situations. For this reason, it is normal procedure in coal liquefaction operations to separate the undesirable finely divided solids prior to further processing of coal liquefaction products.
The procedure for separating the solids from the coal liquefaction solvent phase has been a serious economic disadvantage in coal liquefaction processes. The fine particle size characteristic of the suspended solids creates formidable difficulties in the application of conventional separation techniques such as filtration, centrifugation, and settling.
Existing filtration techniques are unattractive because of slow filtration rates and plugging of filters.
Gravity settling techniques have limited application because of slow settling rates and inefficient ash removal.
Centrifugation techniques for removing the suspended solids have the disadvantage of high equipment and operating costs.
Because of the deficiencies of conventional separation techniques, there remains a need for an effective method for separating finely divided solids from the solvent-rich extract phase of solvated coal either prior or during further upgrading procedures such as catalytic cracking.
Accordingly, it is an object of the present invention to provide an economical method for separating the finely divided suspension of ash solids from the solvent-rich extract phase of a coal liquefaction product.
It is another object of this invention to provide an improved process for converting coal into liquid derivatives having application as fuels.
It is a further object of this invention to upgrade low value refractory petroleum residue from refinery operations into liquid fuel and other high value commercial products.
Other objects and advantages of the present invention shall become apparent from the accompanying drawing and description.